1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a structure of a fringe field switching mode (FFS mode) liquid crystal display device and a method for fabricating the same.
2. Discussion of the Related Art
As society has become increasingly technocentric, demands for various types of display devices have been on the rise. Research has been actively ongoing with respect to flat panel display devices, such as LCDs (liquid crystal displays), PDPs (plasma display panels), ELDs (electro luminescent displays), FEDs (field emission displays), VFDs (vacuum fluorescent displays), as well as other emerging display technologies. Of the aforementioned flat panel display devices, LCDs have received much attention because they are simple to mass produce, light and thin, and can easily be coupled to driving systems that produce high picture quality while consuming small amounts of power.
An LCD device is generally a transmissive display device. More particularly, an LCD device displays a desired image by controlling, pixel by pixel, the amount of light transmitted through a pixel. Pixel control is achieved by individually supplying data signals according to image information to the pixels arranged in a matrix configuration. Such an LCD device is commonly driven by an active matrix (AM) method. In the active matrix method, a switching device such as a thin film transistor (TFT) is added to each pixel. A voltage is applied to liquid crystal molecules of the pixel region through the switching device to drive the liquid crystal molecules in each pixel region.
The LCD device may be classified into various display modes according to the driving characteristics of the liquid crystal molecules. Of the several display modes, a TN (twisted nematic) mode LCD device has generally been used. The TN mode LCD device drives liquid crystal molecules such that an electric field perpendicular to the substrate is turned ON/OFF. Accordingly, orientation of the liquid crystal molecules is at an angle of 0˜90° to a substrate.
Because the liquid crystal molecules in a TN mode LCD device is driven perpendicularly to the substrate, viewing angle characteristics of such a device is not very good. Namely, an angle at which the display can be viewed is limited because color and brightness of an image from a screen varies depending on the direction or angle that a viewer observes the LCD device. To overcome such a disadvantage, a new viewing angle technology, namely, an in-plane switching (IPS) mode LCD device has been proposed.
The IPS mode LCD device generates an in-plane electric field that drives the orientation of the liquid crystal molecules parallel to the substrate along the direction of the in-plane electric field. More particularly, when a voltage is applied to an electrode in an IPS mode LCD device, an in-plane electric field is generated on the substrate and aligns the liquid crystal molecules horizontally, thereby increasing the viewing angle as compared to that of the TN mode LCD device.
FIG. 1 is a schematic plan view illustrating a structure of a unit pixel of an IPS mode LCD device according to a related art. As shown, a unit pixel is defined by a gate line 1 and a data line 3, which are formed of a metallic layer and arranged horizontally and vertically, respectively, on a first substrate of the LCD device. Specifically, an LCD device has (n×m) pixels corresponding to the intersection of n-number of gate lines and m-number of data lines. For purposes of simplifying the explanation of the related art, FIG. 1 shows only one pixel among the (n×m) number of pixels.
At the intersection of the gate line 1 and the data line 3, a switching device, such as a thin film transistor (T) including a gate electrode 1g, a semiconductor layer (not shown), and source/drain electrodes 3a and 3b, is formed. The gate electrode 1g and the source electrode 3a are connected to the gate line 1 and the data line 3, respectively, such that the switching device (T) is turned ON with a signal inputted through the gate line 1 to transmit an image signal supplied through the data line 3 to the unit pixel.
A common electrode line 11 transmitting a common signal is arranged parallel to the gate line 1 within the unit pixel. At least one pair of electrodes driving the liquid crystal molecules, namely, a common electrode 13 and a pixel electrode 15, are arranged in parallel to the data line 3, thereby generating an in-plane electric field parallel to the substrate. Here, the common electrode line 11 and the common electrode 13 are formed simultaneously by extending the common electrode 13 perpendicularly from the common electrode line 11.
The pixel electrode 15 is formed on a passivation layer (not shown) covering an entire substrate. The pixel electrode is arranged near the source/drain electrodes 3a and 3b and is connected to the drain electrode 3b through a contact hole 7. Additionally, a storage capacitor is formed by a storage electrode 11′ extending from the drain electrode 3b and overlapping the common electrode line 11 with an interposing gate insulation layer (not shown) formed therebetween.
On a second substrate (not shown) facing the first substrate, a black matrix for preventing leakage of unnecessary light, a color filter for generating color, and an overcoat layer for flattening are formed. Also, an alignment layer (not shown) for determining an initial direction of alignment for the liquid crystals are is formed on facing surfaces of the first and second substrates. A liquid crystal layer is formed in a gap between the first substrate and the second substrate.
In the IPS mode LCD device having the above-described structure, an in-plane electric field is generated on the substrate because the common electrode 13 and the pixel electrode 15 are all disposed on the same substrate. Accordingly, the liquid crystal molecules in the liquid crystal layer are driven along the in-plane electric field parallel direction to the substrate, namely, the orientation of the liquid crystal molecules is parallel to the substrate. Hence, an image displayed on the front surface of the LCD device can be viewed from any directions, such as from the right, left, lower, and upper sides, thereby fundamentally improving the characteristics of the viewing angle.
One of the disadvantages in the related art IPS mode LCD device is that an aperture area of the LCD device is decreased and the transmittance of light is degraded. The common electrode 13 and the pixel electrode 15 are generally opaque metallic layers disposed in the pixel region where an image is displayed, thereby decreasing the quality of the image. Furthermore, because of the generally opaque nature of the common electrode 13 and the pixel electrode 15, a stronger backlight is needed to provide proper brightness, thereby increasing power consumption. In order to solve the aforementioned problems, using transparent material for the electrode pair has been proposed. However, this solution only slightly improves the aperture ratio, and there is no significant effect on improving light transmittance characteristics of the display.